Paul O. McLaughlin

Optomechanical design of multiscale gigapixel digital camera

Recent developments in multiscale imaging systems have opened up the possibility for commercially viable wide-field gigapixel cameras. While multiscale design principles allow tremendous simplification of the optical design, they place increased emphasis on optomechanics and system level integration of the camera as a whole. In this paper we present the optomechanical design of a prototype two-gigapixel system (AWARE-2) that has been constructed and tested.

Models for the thermal expansion coefficient and temperature coefficient of the refractive index in gradient-index glass

In gradient-index glass the variation of glass composition can also cause a variation in the thermal properties across the gradient region. The difference in thermal expansion and dn/dT across the gradient-index region has been modeled. Several examples illustrate how the models can be utilized to select gradient-index glass compositions that will have constant thermal properties within the gradient region. Experimental measurement of the variations of αL and dn/dT in gradient-index glass is presented in a separate paper.

Design Of A Gradient Index Binocular Objective

The color correction of a gradient index lens was investigated bythe Gradient Index Design Group at the University of Rochester. The object of the analysis was to meet the specifications of an f/3 binocular objective with a 7° field of view consisting of four homogeneous elements. A series of radial gradient singlets, each with a different base glass, was designed to meet these criteria. A study was made of the effects of varying the dispersion of the gradient on the imaging quality of the lens. It was then possible to compute the dispersion that was needed to achromatize the singlet. This calculation was compared with the dispersions in lenses produced by the ion exchange technique and these results will be reported. An extensive tolerance analysis was performed for the various designs. Each coefficient of the index distribution, the surface curvatures, and the thickness were toleranced. Also the tilt and decentration of the gradient within the singlet were toleranced. Methods of tolerancing the dispersion of the gradient were examined and tolerances in terms of relevant parameters were found.

Multimodal imaging system for dental caries detection

Dental caries is a disease in which minerals of the tooth are dissolved by surrounding bacterial plaques. A caries process present for some time may result in a caries lesion. However, if it is detected early enough, the dentist and dental professionals can implement measures to reverse and control caries. Several optical, nonionized methods have been investigated and used to detect dental caries in early stages. However, there is not a method that can singly detect the caries process with both high sensitivity and high specificity. In this paper, we present a multimodal imaging system that combines visible reflectance, fluorescence, and Optical Coherence Tomography (OCT) imaging. This imaging system is designed to obtain one or more two-dimensional images of the tooth (reflectance and fluorescence images) and a three-dimensional OCT image providing depth and size information of the caries. The combination of two- and three-dimensional images of the tooth has the potential for highly sensitive and specific detection of dental caries.

Measurement of the differential thermal expansion and temperature dependence of refractive index in gradient-index glass

The thermal expansion and temperature dependence of the refractive index in gradient-index glass have been investigated. In an earlier paper [ P. O. McLaughlin D. T. Moore , “ Models for the Thermal Expansion Coefficient and Temperature Coefficient of the Refractive Index in Gradient Index Glass,” Appl. Opt.24, this issue ( 15Dec.1985)] these two thermal properties were modeled as functions of the local glass composition within the gradient-index region. In this paper measurements of αL and dn/dT are described. A modified multiple Fabry-Perot interferometer was designed and built into an environmental temperature chamber to measure these thermal properties in gradient-index glass in the 0–100°C range. The interferometer operated under computer control to measure optically the differential thermal expansion and change in refractive-index profile in several samples of gradient-index glass.

Optical performance test and validation of microcameras in multiscale, gigapixel imagers

Wide field-of-view gigapixel imaging systems capable of diffraction-limited resolution and video-rate acquisition have a broad range of applications, including sports event broadcasting, security surveillance, astronomical observation, and bioimaging. The complexity of the system integration of such devices demands precision optical components that are fully characterized and qualified before being integrated into the final system. In this work, we present component and assembly level characterizations of microcameras in our first gigapixel camera, the AWARE-2. Based on the results of these measurements, we revised the optical design and assembly procedures to construct the second generation system, the AWARE-2 Retrofit, which shows significant improvement in image quality.

Efficient testing methodologies for microcameras in a gigapixel imaging system

Multiscale parallel imaging--based on a monocentric optical design--promises revolutionary advances in diverse imaging applications by enabling high resolution, real-time image capture over a wide field-of-view (FOV), including sport broadcast, wide-field microscopy, astronomy, and security surveillance. Recently demonstrated AWARE-2 is a gigapixel camera consisting of an objective lens and 98 microcameras spherically arranged to capture an image over FOV of 120° by 50°, using computational image processing to form a composite image of 0.96 gigapixels. Since microcameras are capable of individually adjusting exposure, gain, and focus, true parallel imaging is achieved with a high dynamic range. From the integration perspective, manufacturing and verifying consistent quality of microcameras is a key to successful realization of AWARE cameras. We have developed an efficient testing methodology that utilizes a precisely fabricated dot grid chart as a calibration target to extract critical optical properties such as optical distortion, veiling glare index, and modulation transfer function to validate imaging performance of microcameras. This approach utilizes an AWARE objective lens simulator which mimics the actual objective lens but operates with a short object distance, suitable for a laboratory environment. Here we describe the principles of the methodologies developed for AWARE microcameras and discuss the experimental results with our prototype microcameras. Reference Brady, D. J., Gehm, M. E., Stack, R. A., Marks, D. L., Kittle, D. S., Golish, D. R., Vera, E. M., and Feller, S. D., “Multiscale gigapixel photography,” Nature 486, 386--389 (2012).

Optical design of a multimodal imaging system

A multimodal imaging system has been developed for tooth tissue imaging. This imaging system is designed to obtain one or more two-dimensional images of the tooth tissue, and those two-dimensional images are rendered with advanced algorithms to provide a high-contrast image. This system combines polarized reflectance imaging, fluorescence imaging, and optical coherence tomography (OCT) imaging. The imaging system design, as well as some experimental results, will be discussed in the presentation.

Liquid crystal lens focusing in monocentric multiscale imagers

In multiscale imagers a single objective lens is shared by multiple secondary optical systems, so that a high-resolution wide-angle image is acquired in overlapping fields sensed by multiple conventional focal planes. In the “AWARE2” 2 Gigapixel imager, F/2.4 optics cover a 120 degree field of view using a monocentric glass primary lens shared by 221 molded plastic subimagers, each with a 14 Megapixel focal plane. Such imagers can independently focus parts of the image field, allowing wide-angle imaging over relatively close and deep image fields. However, providing hundreds of independent mechanical focus adjustments has a significant system impact in terms of complexity, bulk, and cost. In this paper we explore the use of an electronically controlled liquid crystal lens for focus of multiscale imagers in general, and demonstrate use with the AWARE2 imager optics. The Lens Vector Auto Focus (LVAF) liquid crystal lens provides up to 5 diopters of optical power over a 2.2mm aperture diameter, the maximum currently available aperture. However, a custom lens using the same materials and basic structure can provide the 5 diopters power and 6.4 mm aperture required to obtain full resolution overlapping image fields in the AWARE2 imager. We characterize the LVAF lens and the optical performance of the LVAF lens in the current AWARE2 prototype, comparing the measured and optically modeled resolution, and demonstrating software control of focus from infinity to an 2m object distance.

Considerations for tolerancing aspheric optical components

Optical designs often specify both surface form and centering (tilt and lateral displacement) tolerances on aspheric surfaces. Such tolerances are often ambiguous and current standards do not resolve this ambiguity. We describe several difficult issues that arise in attempting to interpret aspheric centering tolerances and their consequences for optical design and metrology. We conclude by calling on the optics community to standardize an interpretation method for aspheric tolerances.